1. Field of the Invention
The invention relates to the field of semiconductor devices, and in particular to a method and structure for preventing excess base-collector leakage currents in heterojunction bipolar transistors.
2. Relevant Art
A heterojunction bipolar transistor (HBT) is a bipolar transistor that includes at least two semiconductor materials that have different bandgap values. The bandgap differential controls the flow of minority carriers within the HBT to provide the desired transistor functionality. By using different semiconductor materials, HBTs can overcome the performance limitations associated with conventional single-material bipolar junction transistors (BJTs).
FIG. 1A shows a cross sectional view of a conventional HBT 101. HBT 101 includes a subcollector 111 formed on a substrate 100, a collector 121 formed on subcollector 111, a base 131 formed on collector 121, and an emitter 141 formed on base 131. The top of emitter 141 includes an emitter cap 151 for improved electrical conductivity to emitter 141.
HBT 101 is in a “wired” state, with interconnects 113, 133, and 153 providing electrical connectivity to contacts 112, 132, and 153, respectively, formed on subcollector 111, base 131, and emitter cap 151.
HBT 101 can either be a single heterojunction bipolar transistor (SHBT) or a double heterojunction bipolar transistor (DHBT), depending on the material used for the various transistor components. For example, an SHBT could comprise an indium gallium arsenide (InGaAs) base 131 and collector 121, and an indium phosphide (InP) emitter 141, thereby forming a heterojunction between the emitter and the base. A DHBT could be formed by changing collector 121 from InGaAs to InP, thereby creating another heterojunction between base 131 and collector 121. The second heterojunction allows additional bandgap engineering to be performed to further enhance device performance.
An oxide (SiO2) passivation layer 160 is formed over the exterior surfaces of HBT 101 (i.e., the surfaces of emitter 141, base 131, collector 121, optional emitter cap 151, and subcollector 111 not in contact with each other). Nitride (Si3N4) is also commonly used as a passivation layer. Passivation layer 160 protects HBT 101 from damage and contamination.
Another critical function provided by passivation layer 160 is to minimize problematic base-collector (B-C) surface leakage currents. B-C surface leakage currents arise when dangling (unpassivated) bonds on the exterior surfaces of base 131 and collector 121 create current paths along those exterior surfaces. The B-C surface leakage can result in excess power consumption and degraded device performance for HBT 101.
Unfortunately, conventional passivation layers, such as passivation layer 160, are not optimized for HBTs. As is known in the art, the use of silicon dioxide or silicon nitride to passivate an HBT allows B-C surface leakage currents to increase as the HBT is exposed to temperature cycling (see H. Wang, et al., Proc. 13th IPRM, May 2001, pp. 252-255 and T. Kikaws et al., Proc. 10th IPRM, May 1998, pp. 76-79.). This B-C surface leakage current increase is due in large part to thermally induced strain-related defects that form in the bond between the passivation layer and the underlying HBT components.
FIG. 1B is a representation of a B-C passivation portion 161 of passivation layer 160 shown in FIG. 1A. The atoms forming base 131 are depicted as white dots in a base lattice 131-L. The underlying atoms forming collector 121 are depicted as black dots in a collector lattice 121-L. And the atoms forming passivation layer 160 are depicted as white squares in a passivation layer lattice 160-L. The atoms in base lattice 131-L and the atoms in collector lattice 121-L are all spaced by the same bond length (“lattice constant”), and so base lattice 131-L and collector lattice 121-L are “lattice matched”. Typically, the materials in the base, collector, and emitter of an HBT must be lattice-matched to ensure good performance.
However, an oxide or nitride layer will generally have a much smaller lattice constant than the materials used to form the HBT components. As shown in FIG. 1B, the atoms in passivation layer lattice 160-L are more closely spaced than the atoms in base lattice 131-L and collector lattice 121-L, resulting in “lattice mismatch”.
Lattice mismatch creates a significant strain at the interface between the mismatched materials, so that thermal cycling can cause bond dislocations and the dangling bonds that provide pathways for surface B-C leakage currents. Thus, conventional passivation techniques result in reduced HBT lifetime and degraded HBT performance.
Accordingly, it is desirable to provide a method and structure for passivating HBTs that minimizes B-C surface leakage currents even after the HBT is exposed to high temperatures.